Figure 1.

Characterization of the synthesized UCNPs. (a) SEM and (b) high-angle annular dark-field (HAADF) scanning TEM images of β-NaYF₄: Yb³⁺,Tm³⁺/NaYF₄ core/shell nanoparticles. (c) Spectra of the UCNPs in chloroform irradiated with 975-nm laser light with the intensity of 3.5, 7 and 11 W cm⁻². (d) Integral conversion efficiency of UCNPs versus the excitation intensity at 975 nm measured using the calibrated integrating sphere setup. The saturation was reached at ∼20 W cm⁻². The magenta line is provided as a guide to the eye.